1. Field of the Invention
The present invention relates to a charging control apparatus for an internal combustion engine with a dual turbocharger system which can suppress a decrease in charging pressure during a transition from "one-turbocharger-operation" to "two-turbocharger-operation."
2. Description of the Related Art
An internal combustion engine with a dual turbocharger system is shown in Japanese Patent Publication HEI 3-138,420 (which corresponds to U.S. Pat. No. 5,081,842).
The dual turbocharger system includes a first turbocharger operated at all intake air quantities and a second turbocharger operated solely at large intake air quantities. More particularly, an intake switching valve is installed in a portion of the intake conduit located downstream of the compressor of the second turbocharger and an exhaust switching valve is installed in a portion of the exhaust conduit located downstream or upstream of the turbine of the second turbocharger. When the intake switching valve and the exhaust switching valve are closed, operation of the second turbocharger is stopped and only the first turbocharger is in operation. When the intake switching valve and the exhaust switching valve are open, both the second turbocharger and the first turbocharger are in operation. To make the switch from "one-turbocharger-operation" to "two-turbocharger-operation" smooth, an exhaust bypass conduit is provided to bypass the exhaust switching valve and an exhaust bypass valve is installed in the exhaust bypass conduit. Before the intake switching valve and the exhaust switching valve are opened, the exhaust bypass valve is opened so that the second turbocharger is preliminarily rotated or run-up. In the conventional turbocharged engine, the switching timing from "one-turbocharger-operation" to "two-turbocharger-operation" and the duration of the running-up of the exhaust bypass valve are a constant with respect to the engine speed or intake air quantity.
However, there are the following problems with the conventional turbocharged engine:
(a) The running-up characteristic of the second turbocharger varies in accordance with the engine speed or intake air quantity. Therefore, if the duration of the running-up of the second turbocharger is fixed so as to be optimum for high engine speeds, the duration of the running-up is too short for a low engine speed and the torque shock during the transition from "one-turbocharger-operation" to "two-turbocharger-operation" will be large at low engine speeds. PA0 (b) On the other hand, if the duration of the running-up of the second turbocharger is fixed so as to be optimum for low engine speeds, the duration of the running-up is too long for high engine speeds and an overrun of the first turbocharger will occur at the switching from "one-turbocharger-operation" to "two-turbocharger-operation." PA0 (c) When the engine speed or intake air quantity exceeds the predetermined value for switching from "one-turbocharger-operation" to "two-turbocharger-operation", the exhaust switching valve will be delayed in opening as a result of various mechanical delays and the exhaust gas flow resistance will be increased and the engine output will be suppressed until the exhaust switching valve is completely open. Since the mechanical delay is constant, independent of a change of the engine load, the engine output will be seriously affected by the exhaust gas flow resistance during rapid acceleration at high engine speeds. PA0 (d) Because the operation of the conventional turbocharged engine including start of the exhaust bypass valve and switching from "one-turbocharger-operation" to "two-turbocharger-operation" is based on fixed engine speed or intake air quantity, the intake air quantity can reach a predetermined intake air quantity for switching too quickly when the gear shift position is at low positions. As a result, at low gear shift positions, the running-up of the exhaust bypass valve is insufficient and a relatively large torque shock tends to occur during switching from "one-turbocharger-operation" to "two-turbocharger-operation".